The predictability and manageability of singular phenomena is low. Their impacts can be sudden, large, and irreversible on a time scale of centuries. Regularizing such impacts would be an appropriate response, but this would require much better understanding of the statistics and characteristics of the complex processes involved. The presence of singularities therefore makes analytic and political treatment of the climate change problem particularly difficult.
Little is known, in quantitative terms, about the potential damages that could be inflicted by singularities on ecosystems and market sectors across the globe. This deficit has two main reasons (see also Moss and Schneider, 2000). First, extensive research on the causes, mechanisms, and impacts of singular events in the context of climate change is just getting started. Second, mechanistic and probabilistic analysis of complex nonlinear systems is more demanding—by orders of magnitude—than investigation of simple linear ones.

Figure 19-6: Stability of North Atlantic thermohaline circulation (THC) computed with the CLIMBER model (Petoukhov et al., 2000). Degree of shading indicates probability of THC collapse. Light shading denotes low probability; dark shading denotes high probability. The higher the hydrological sensitivity (HHS = high hydrological sensitivity, LHS = low hydrological sensitivity), the faster the rate of temperature increase, or the greater the magnitude of temperature increase, the more likely that the North Atlantic THC becomes unstable.

The knowledge base for assessing consequences of singularities will probably be broadened considerably over the next 5-10 years. Further advances in simulation modeling soon will allow better projections of future climate variability down to modified extreme events statistics (CLIVAR, 1998), as well as better translations of those projections into impacts on natural and societal systems (e.g., Weyant et al., 1996; Alcamo et al., 1998; Rotmans and Dowlatabadi, 1998). Earth system analysis, as supported by the big international research programs—World Climate Research Programme (WCRP), International Geosphere-Biosphere Programme (IGBP), and International Human Dimensions Programme (IHDP)—will bring about more complete understanding of macro-singularities within the responses of the Earth system under pertinent forcing (Schellnhuber, 1999). A major source of information and comprehension, in this context, will be evidence provided by paleorecords (IGBP, 1998). These scientific efforts should assist the decisionmaking process by creating a clearer picture of the future. Unfortunately, creating plausible projections is always tricky in practice (Sarewitz et al., 2000).

A major challenge is to make responsible use of available information regarding the likelihood and the consequences of conceivable singular events. Responsibility here means the obligation of decisionmakers to make the "right" decision, taking into account the diverse societal values and wide ranges of individual interests that are at stake and that may be mutually contradictory. Thus, the standard challenge is to develop proper policies under uncertainty (i.e., neither ignorance nor omniscience) to achieve the objectives of the UNFCCC and to satisfy affected stakeholders as well as possible.

A broad and intensive discourse on the ethical aspects of singular responses to climate change (e.g., Markandya and Halsnaes, 2000; Munasinghe, 2000; Toth, 2000) is rediscovering many of the arguments put forward in traditional moral philosophy and risk policy. Ethical and procedural aspects of this type have been examined in various other contexts before, where certain concepts (such as human rights) act as a constraint on economic activity (emphasizing utilitarian goals), even when the cost-benefit ratio is unfavorable (e.g., the review of the agricultural situation by Aiken, 1986).

One of the crucial questions is how to deal with high-consequence impacts that may wipe out entire systems or cultures. Such non-implausible "nightmare" or "doomsday" scenarios could result from the speculative but consistent concatenation of individually possible causal relationships (e.g., Schellnhuber and Yohe, 1997). A vexing question is whether the lack of credible scientific evidence for such a scenario provides justification to ignore its possibility completely. Some argue that such effects have to be avoided by all means, irrespective of the economic burdens involved. Others argue that the uncertainties involved do not provide enough support for extensive measures and their economic costs. Within the climate-change framework, however, many incalculable risks could be reduced considerably by more sensible measures. The debate on the "legitimacy" of the different perspectives is impossible to resolve, however (Jasanoff, 1990).

The vague evidence provided by the present state of research supports the notion that even relatively small changes in mean climate could lead to large changes in the occurrence of stochastic extreme events. Furthermore, it suggests that large-scale discontinuities are unlikely below a 2°C warming but relatively plausible for a sustained warming of 8-10°C. The relatively small set of investigations discussed above lead to the conclusion that a warming range of 4-5°C seems to represent a critical disturbance regime where macro-discontinuities may start to emerge. This temperature threshold appears to be sensitive to the rate of change at which this level is reached.

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